1. Field of the Invention
The invention relates to a fax transmission system which uses both digital communications lines and conventional analog communications via public switched telephone network to determine an appropriate path for a fax transmission. In particular, the invention relates to a point-of-presence (POP) which is used to packetize fax data received via analog telephone lines, and then send the packetized data over a digital communications line to another POP, which then unpacketizes the data to be sent to a destination fax machine. In the above-described system, virtual POPs are used to communicate with devices on other networks.
2. Description of the Related Art
When a facsimile is transmitted using conventional systems, the data is transmitted entirely through voice grade, analog telephone lines. A typical facsimile transmission route is shown in FIG. 1. A source facsimile device 10 dials a phone number for a destination facsimile device 12. The call is routed through a telephone jack 20 to a local exchange carrier at a central office (LEC CO) 30. Subsequently, a connection is made from the LEC CO 30 to a local inter-exchange carrier's point of presence (IXC POP) 40. The local IXC POP 40 makes a connection through a long distance carrier facility 50 to a destination IXC POP 42 which in turn makes a connection to a destination LEC CO 32. The destination LEC CO 32 then rings the phone number for the destination facsimile device 12. Once the destination facsimile device 12 answers, an end-to-end connection is established between the two facsimile devices 10, 12. The appropriate facsimile session setup is then performed by the two facsimile devices 10, 12, and is followed by the facsimile data transmissions. Upon completion of the facsimile transmission, the facsimile devices hang-up and the connection is terminated.
In FIG. 1, the connection between the two LEC CO's 30, 32 is illustrated as being through a cloud 60. The cloud 60 is conceptually nothing more than a mechanism by which a source LEC CO 30 communicates with a destination LEC CO 32. How this is accomplished is not important other than to point out that it is a network of communications which allows point to point communications for devices on the network. The communications medium can be copper wire, fiber optic cable, satellite link, or microwave link. It behaves as a virtual wire from one place to another.
In the conventional system as illustrated in FIG. 1, when one facsimile device dials another, a circuit-like connection is created between the two devices. Data is then transmitted between the devices in real-time. In other words, there is virtually no delay from the time when the source facsimile device 10 sends data to the time when the destination facsimile device 12 receives the data.
Further, in the conventional system, telecommunications services (phone lines, etc.) used for facsimile transmissions, and their associated cost structure, are formally defined by the carrier in a document called a tariff. Tariffs are filed for approval by state regulatory commissions and/or the Federal Communication Commission (FCC).
There are also store and forward services available for transmitting facsimile information. With store and forward, the facsimile information is generally transmitted to a local vendor with information about the final destination. The local vendor then takes responsibility for delivering the facsimile. Such services are not real-time in that there is no circuit-like connection between the source and destination facsimile devices. In such systems, the facsimile transmission is usually sent at a later time when the rates are less. A disadvantage of such systems is that in the event a facsimile cannot be delivered to the destination facsimile device, there may be no reliable way of notifying the user of the source facsimile device that the facsimile has not been delivered. Important facsimiles requiring immediate delivery and confirmation of receipt generally cannot be sent through such systems.
As described above, facsimile transmissions today are accomplished through the use of analog grade circuits. The bandwidth of analog voice grade circuits is however very small and so telephone companies have migrated their equipment to provide their transmission services using a digital network. Nevertheless, access to the digital network is still made through voice grade lines and thus facsimile transmissions still need to be converted from analog to digital. To faithfully reproduce an analog voice transmission requires a digital bandwidth of 56 Kbps. Thus, 56 Kbps of bandwidth must be reserved for every voice chanel to be carried over the digital medium. Utilizing an analog signal, facsimile over a voice grade line is currently limited to sending the equivalent of 14.4 Kbps, thereby wasting a minimum of 41.6 Kbps. As a consequence, when a facsimile is sent over a voice grade circuit through the network's digital transmission facilities, most of the bandwidth is not utilized even though it is being paid for.
An example of the above-described digital network is shown in FIG. 2, and corresponds to a system recommended in a document published by The International Telecommunications Union (ITU, formerly "CCITT"). In the ITU system, data is transmitted digitally between a source facsimile packet assembly/disassembly facility (FPAD) 44 and a destination FPAD 46 through a Packet Switched Public Data Network (PSPDN) 51. The access to either FPAD 44 or 46 is still made through its respective General Switched Telephone Network (GSTN) 31 or 33. The CCITT system has some technical characteristics, however, which make it unattractive, particularly for facsimile transmissions. They are:
1. It runs on X.25 protocol which significantly increases the length of call. PA1 2. It does not guarantee delivery of data. PA1 3. It has a variable delay in delivery of communicated packets. PA1 4. It does not accommodate switched analog access lines.